After natural gas has been removed from the ground, the gas stream is transported from place-to-place via pipelines. It is desirable to know with accuracy the amount of gas flowing in the stream, and particular accuracy is demanded when the fluid is changing hands, or “custody transfer.” Ultrasonic flow meters may be used to measure the amount of natural gas flowing in a pipeline, and ultrasonic flow meters have sufficient accuracy to be used in custody transfer.
In an ultrasonic flow meter, acoustic signals are sent back and forth across the gas stream to be measured. Based on parameters of received acoustic signals, the gas flow velocity at several distinct elevations in the flow meter is determined. Based on the flow velocities and the known cross-sectional area of the flow meter, the gas flow volume may be calculated.
However, in some cases liquids accumulate in the lower portion of the flow meters measuring natural gas stream. The liquids may be hydrocarbons or water. For example, depending on pressure and dew point, hydrocarbons may precipitate out of the natural gas stream, causing liquid accumulations. As yet another example, pipelines may be hydrostatically tested by filling the lines with water, and in some cases the water is not fully removed before the pipeline carries natural gas. Regardless of the nature of the liquid, liquid accumulations reduce the cross-sectional area of the flow meter, and the reduced cross-sectional area has double effect on flow volume measurements. For a constant actual flow volume through a meter, a reduced cross-sectional area results in increased measured flow velocity. Moreover, the flow meter assumes a cross-sectional area, and determines a flow volume based on measured flow velocity and cross-sectional area. Liquid accumulations reduce the actual cross-sectional area, and thus actual volumes will be less than measured volumes based both on the reduced cross-sectional area and the tendency for the gas flow velocity to increase in the presence of the reduced cross-sectional area.
Some related art ultrasonic flow meters attempt to determine whether liquid has accumulated in the flow meter by way of a transducer pair that directs acoustic signals toward the lower-most portion of the meter. When no liquid is present, an acoustic signal created by one transducer traverses the meter, reflects from the lower-most portion of the meter, and then propagates to the second transducer. However, when liquid is present, the acoustic signal intersects and reflects from the surface of the liquid, rather than the lower-most portion of the meter, and thus the path length for the acoustic signal changes. Parameters of the acoustic signal indicate the changed path length, and thus the presence of liquid. However, a dedicated transducer pair used for liquid detection increases the cost and complexity of the flow meter, and retrofitting existing meters (whose acoustic signals traverse the flow meter substantially horizontally), though theoretically possible, is prohibitively expensive. Thus, systems and methods to determine whether liquid is present in acoustic meters would be beneficial.